Transmission line-to-waveguide transitions are used extensively in microwave communications systems such as radar and satellite systems. The systems may include a waveguide antenna for phased array applications or a conventional waveguide of arbitrary cross-section. In these systems the microwave signal may be bi-directionally coupled between a waveguide and a transmission line with minimal power (insertion) loss and maximum signal clarity.
One example of a known waveguide-based spatially combined amplifier is shown in FIG. 1(a). Microwave power is incident from the waveguide structure on the left. This energy illuminates a two-dimensional array of several slotline modules, with each slotline module forming a column of the array. Each slotline module consists of a dielectric card, upon which at least two circuit elements are mounted. The input energy is coupled to these circuit elements through an antenna which tapers to a slotline transmission line. Structures transform the microwave energy from the slotline mode to a microstrip mode, with the microstrip conductor printed on the side of the card opposite to the slotline. Energy on the microstrip is coupled to the input of the circuit elements. The outputs of the circuit elements are coupled to the waveguide in a similar manner.
Another known waveguide-based spatially combined amplifier is shown in FIG. 1(b). In FIG. 1(b), microwave power is incident from the coaxial waveguide structure on the left. This energy illuminates a two-dimensional annular array of several slotline modules, with each slotline module forming a radial column of the array. Each slotline module consists of a dielectric card, upon which at least two circuit elements are mounted. The input energy is coupled to these circuit elements through an antenna which tapers to a slotline transmission line. Structures transform the microwave energy from the slotline mode to a microstrip mode, with the microstrip conductor printed on the side of the card opposite to the slotline. Energy on the microstrip is coupled to the input of the circuit elements. The outputs of the circuit elements are coupled to the output coaxial waveguide in a similar manner.
The use of the word slotline is intended to include any and all of the family of balanced microwave transmission line structures where the signal power is concentrated in a gap between two substantially symmetric conductors printed on one or both sides of a dielectric substrate. Common terms for these transmission line structures include slotline, finline, antipodal finline, unilateral finline, bilateral finline, and insulated finline. The use of the term “slotline” in this application is therefore intended to be consistent with standard terminology widely known in the art.
In existing rectangular waveguide spatial power combiner configurations such as that of FIG. 1(a), the field intensity in the rectangular waveguide follows a sinusoidal distribution, with the result that the slotline modules in the center of the structure receive more power than the slotline modules along the edge. Similarly, the outputs of the central modules couple to the waveguide more effectively than the edge modules. This imbalance in the signal amplitude reduces the power-combining efficiency of the entire array.